1. Technical Field of the Invention
The present invention relates to a liquid sensor and a liquid container having the liquid sensor, and more particularly, to a mounting structure of a liquid sensor and a liquid container which are suitable for detecting the amount of remaining liquid in a liquid jetting apparatus.
2. Description of the Related Art
As a representative example of a conventional liquid jetting apparatus, there is known an inkjet printer including an inkjet printing head for printing an image. Examples of other liquid jetting apparatuses can include an apparatus including a color-material jetting head which is used for manufacturing a color filter of a liquid crystal display or the like, an apparatus including an electrode-material (conductive paste) jetting head which is used for forming an electrode of an organic EL display, a field emission display (FED), or the like, an apparatus including a living-organic-material jetting head which is used for manufacturing a biochip, and an apparatus including a sample jetting head as a precision pipette.
In the inkjet printer which is the representative example of the liquid jetting apparatus, a carriage is mounted with an inkjet printing head having a pressurizing unit which pressurizes a pressure generating chamber and a nozzle opening for ejecting pressurized ink in ink droplets.
The inkjet printer has a structure that a printing work can be continuously performed by continuously supplying the ink in an ink container to the printing head through a flow path. The ink container is embodied as a detachable cartridge which can be replaced by a user, for example, when the ink is consumed up.
Conventionally, as a management method for ink consumption of the ink cartridge, there are known a management method in which the number of ink droplets ejected from the printing head or the amount of ink absorbed by a maintenance is integrated in software to calculate the amount of consumed ink and a method of managing the time when a predetermined amount of ink is actually consumed by mounting an electrode for detecting a liquid level in the ink cartridge.
However, the management method, in which the number of ejected ink droplets or the amount of ink is integrated in software to calculate the amount of consumed ink, has the following problems. Some heads show a weight variation in ejected ink droplets. The weight variation in ink droplets dose not have an effect on display quality. However, in consideration of the time when errors in the amount of consumed ink due to the variation are accumulated, the ink cartridge is filled with the ink together with margin ink. Accordingly, depending upon the individual ink cartridges, ink is left as much as the margin ink.
On the other hand, in the method of managing the time when the ink is consumed by the use of an electrode, the actual amount of ink can be detected, thereby managing the amount of remaining ink with high reliability. However, there are drawbacks in that kinds of ink to be detected are limited and a seal structure for an electrode becomes complicated, because detection of an ink level relies on conductivity of ink. In addition, precious metal having excellent conductivity and corrosion resistance is generally used as a material of the electrode. Therefore, manufacturing cost of the ink cartridge runs up. Further, since two electrodes need to be mounted, the number of manufacturing processes is increased, thereby increasing the manufacturing cost.
Accordingly, a device developed for solving the above-mentioned problems is disclosed as a piezoelectric device in JP-A-2001-146024 or JP-A-2001-146030. Such a piezoelectric device can reliably detect the amount of remaining liquid and does not require a complex sealing structure. The piezoelectric device can be mounted on a liquid container for use.
That is, in the piezoelectric device described in the publications, when ink exists or does not exist in a cavity facing a vibration portion of a piezoelectric device, it is possible to monitor the amount of ink remaining in an ink cartridge by the use of variation in resonance frequency of a residual vibration signal resulting from residual vibration (free vibration) of the vibration portion after compulsory vibration with driving pulses.
FIG. 27 shows an actuator constituting the above-described conventional piezoelectric device. The actuator 1106 includes a substrate 1178 having a circular opening 1161 substantially in its center, a vibration plate 1176 displaced in one surface (hereinafter, referred to as ‘front surface’) of the substrate 1178 so as to cover the opening 1161, a piezoelectric layer 1160 disposed on the front surface of the vibration plate 1176, an upper electrode 1164 and a lower electrode 1166 interposing the piezoelectric layer 1160 therebetween, an upper electrode terminal 1168 which is electrically connected to the upper electrode 1164, a lower electrode terminal 1170 which is electrically connected to the lower electrode 1166, and an auxiliary electrode 1172 which is arranged between the upper electrode 1164 and the lower electrode 1168 to electrically connect both of them.
Each of the piezoelectric layer 1160, the upper electrode 1164, and the lower electrode 1166 has a circular portion serving as a main body. The respective circular portions of the piezoelectric layer 1160, the upper electrode 1164, and the lower electrode 1166 constitute a piezoelectric element.
The vibration plate 1176 is formed on the front surface of substrate 1178 so as to cover the opening 1161. The vibration region, which is actually vibrated in the vibration plate 1176, is defined by the opening 1161. The cavity 1161 is formed by the part of the vibration plate 1176 facing the opening 1161 and the opening 1161 of the substrate (cavity forming member) 1178. The surface (hereinafter, referred to as ‘back surface’) of the substrate 1178 opposite to the piezoelectric element faces inward to the ink container. Accordingly, the cavity 1162 is constructed so as to be brought into contact the liquid (ink). In addition, the vibration plate is liquid-tight to the substrate 1178 so that liquid does not leak in the front surface of the substrate 1178 even though liquid enters the cavity 1162.
In the actuator 1106 according the above-described conventional technique, the residual vibration (free vibration) of the vibrating portion which is generated after the vibration portion is forcefully vibrated by the vibrating pulse applied to the piezoelectric element, is detected as a back electromotive force by the same piezoelectric element.
As shown in FIG. 28, the above-described conventional actuator (piezoelectric device) 1106 is mounted on the container wall of a container body 1181 of the ink cartridge 1180 and is constructed so that the cavity 1162 for receiving ink to be sensed is exposed to the ink reservoir space within the ink cartridge 1180.
However, the above-described conventional actuator (piezoelectric device) 1106 is constructed so that the cavity 1162 is exposed to the ink reservoir space within the ink cartridge 1180, as described above. Therefore, if the ink within the ink cartridge 1180 is foamed by a vibration or the like, bubbles easily enter the cavity 1162 of the actuator 1106. If the bubbles enter the cavity 1162 and accumulated therein, the resonant frequency of the residual vibration detected by the actuator 1106 becomes high so that it is erroneously determined that the liquid level passes through the position of the actuator 1106 and the amount of remaining ink becomes small, despite the fact that the amount of ink remaining in the ink cartridge 1180 is adequate.
Since the actuator 1106 is constructed so as to expose the cavity 1162 to the ink reservoir space within the ink cartridge 1180, the ink pressure within the ink reservoir space has a direct effect on the vibration plate 1176 and piezoelectric layer 1160 of the actuator 1106. Therefore, in the ink cartridge 1180 or the like, the ink within the ink reservoir space is severely vibrated by the carriage reciprocating on a printing operation, so that the ink pressure generated by the vibration has a direct effect on the actuator 1106, which results in erroneous determination.
In addition, it is also considered that a barrier for preventing a vibration or a wave of ink is formed in the position facing the actuator 1106. However, the space structure around 1106 becomes complicated and the vibration mode of the residual vibration detected by the actuator 1106 also becomes complicated as much. As a result, sensing becomes hard so as to dull detection sensitivity.
When the size of cavity 1162 of the actuator 1106 is made small so that the passing time of the ink level is precisely detected, the meniscus of ink is easily formed inside the cavity 1162. For this reason, even though liquid level has passed through the position of the cavity 1162 due to ink consumption, it is erroneously determined that the liquid level has not passed through the position of the cavity 1162 and the amount of remaining ink is adequate, because ink is accumulated in the cavity 1162.
When the sensor unit disclosed in JP-A-2001-146024 or JP-A-2001-146030 is used, the ink is allowed to freely enter the cavity opposed to the vibration plate, but the ink is not allowed to enter the space in which the piezoelectric element and the like as electrical elements are disposed. Accordingly, the spaces between the adjacent elements should be liquid-tightly sealed at the time of mounting.
As the sealing structure, there is known a structure that the sensor unit is bonded directly to the circumferential edge of an opening of the container body or a structure that the sensor unit is bonded directly to the circumferential edge of an opening of a module and then the module is mounted on the container body with an O ring therebetween. However, since the sensor unit is bonded to the circumferential edge of the opening, deviation in size makes it difficult to secure the sealing ability. In addition, when the sensor unit is bonded directly to the circumferential edge of the opening of the container body or the circumferential edge of the opening of the module, it can be easily affected by a wave motion of the ink or bubbles in the ink, thereby causing erroneous detection.